You may not have heard much of anything about it, but millions of New Yorkers who use the Big Apple’s underground public transit system recently become the victims of a major chemical attack — no, not an attack by some unknown terrorist cell from the Middle East, but one initiated by the New York Police Department (NYPD).

According to new reports, the NYPD, in conjunction with a prominent federally-run laboratory, blasted plumes of a chemical gas known to cause respiratory problems and lung injury into the New York City subway system as part of a coordinated “attack drill.” The purpose of the drill, as officially stated, was to assess how terrorists might try to disburse more harmful chemicals into a public place as part of a future biological attack.

After placing a number of air-sampling devices both inside subway tunnels and at ground-level entryways all around the city, NYPD officers working together with officials from the Brookhaven National Laboratory in Long Island proceeded to pump tracer gases known as perfluorocarbons into subway tunnels during people’s morning commute. They then tested how the gases spread throughout the day.

NBC New York explains that the chemical gas, which is derived from the waste of aluminum processing plants, was sprayed into subway tunnels at such low concentrations that nobody was expected to become injured by them, at least according to the so-called experts. In fact, most of the mainstream media sources reporting the story have declared the gas to be “safe.”